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Chapter 7

Chapter 7. Gravity. How Does It Happen?. There is an old tale about Newton sitting under an apple tree and watching an apple fall Newton also realized that the apple and even the moon were affected by the Earth’s gravity

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Chapter 7

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  1. Chapter 7 Gravity Presented by April Senger

  2. How Does It Happen? • There is an old tale about Newton sitting under an apple tree and watching an apple fall • Newton also realized that the apple and even the moon were affected by the Earth’s gravity • So why does an apple fall straight down and the moon appears to stay in a set position? Presented by April Senger

  3. What Makes Gravity Strong? • Gravity is dependent on two factors…distance between the objects and mass • Gravity is directly proportional to mass • The distance is inversely proportional to gravity Presented by April Senger

  4. Concept Check • What happens to the force acting between two bodies if the mass of one body is doubled? • The force doubles • What happens if the mass of the other body is doubled? • Same • What happens if the masses of both the bodies are doubled? • The forces is 4 x as much • What happens if the mass of one body is doubled and the other is tripled? • It would increase by 6 x Presented by April Senger

  5. Details on Distance • Not only are gravity & distance inversely proportional, they follow the Inverse-Square Law • This means that if I double the distance, the gravity decreases by ¼ • If I triple the distance, the gravity decreases by 1/9 • What happens if you quadruple the distance? • 1/16 the force of gravity • A spray can and its paint, light from a match, radiation from a piece of uranium, and sound from a cricket all follow this same law Presented by April Senger

  6. Centering Our Thoughts • Remember that gravity is the force that pulls mass towards the CENTER of the Earth • Most people assume that if you throw a ball up, it comes down because the Earth’s surface pulls it there • Try digging a hole and see what happens…The pull is actually towards the center of the Earth Presented by April Senger

  7. Concept Check • How much does the force of gravity change between the Earth and a receding rocket when the distance doubles? Triples? Ten times as much? • ¼, 1/9, 1/100 • Consider an apple at the top of a tree. The apple is pulled by the Earth’s gravity with a force of 1 N. If the tree were twice as tall, would the force of gravity by only ¼ as strong? • Don’t forget the pull is towards the center of the Earth. Doubling a tree doesn’t compare to the 6,370 km radius of our planet. The apple would weight nearly the same Presented by April Senger

  8. No Escaping It • As long as two objects have mass, they will always have gravity • Think of your Lucky Charms floating in the milk • Given time, they will clump and cling to the sides • Even two dust particles have gravity Presented by April Senger

  9. Concept Check • Light from the sun obeys the inverse-square law. If you were on a planet twice as far from the sun, how bright would the sun look? • ¼ as bright • How bright would the sun look if you were on a planet twice as close to the sun? • 4 x as bright Presented by April Senger

  10. Ocean Tides • Why does the Earth have tides? • In order to have the amount of water in the ocean move that much, there must be a large mass or masses near by • The moon rotates to the same position every 24 hrs and 50 minutes so tides are never exactly the same time each day • The moon is much less massive than the Sun • The sun does pull on our oceans but doesn’t have the impact that the moon has…Why? • The differences in pull on the ocean are what cause the tides Presented by April Senger

  11. Concept Check • If the moon didn’t exist, would the Earth still have tides? If so any idea how often? • Yes there would be due to the sun. They would occur twice a day every 12 hours • We know that both the moon and the sun produce our ocean tides. The moon plays a greater role because it is closer. Does its closeness mean it pulls the ocean with more g force than the sun? • No. Tides are not caused by gravitational pulls but rather differences in the gravitational pull. If the moon was closer, the difference in pulls would be too much and it would tear the moon to pieces. This is what some think caused the rings of Saturn Presented by April Senger

  12. During a new or full moon the Sun, Earth and moon are all lined up This causes the highest tides called spring tides (nothing to do with the season) When the moon is half way through a phase, the Sun and moon’s pulls partially cancel each other producing a neap tide Why don’t lakes have tides? Full Moons? Presented by April Senger

  13. Good Tidings for All • Tides can occur in the atmosphere and magma called Earth Tides • For the purpose of this class we ignore the tilt of the Earth, landmasses, friction with ocean bottoms, and other factors that influence tidal motion • It is a complex and fascinating occurrence Presented by April Senger

  14. Gravity and the Universe • We will spend more time studying the origin of the Universe in another chapter but consider this… • If everything that has mass has gravity, won’t we all eventually pull ourselves back into one central location? • Some believe in a theory called the Big Bang but in the future we might experience the Big Crunch when all material centralizes • Scientists that believe in the Oscillating Universe theory feel that it would take about 100 billion years for the process to occur Presented by April Senger

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